1. Field of the Invention
The present invention relates to a rubber composition and to a pneumatic tire using the rubber composition, and more particularly, to a rubber composition and to a pneumatic tire using the rubber composition in which driving and braking abilities on snow or icy road surface (hereinafter referred to only as "snow/ice grip performance") are improved and abrasion resistance and processability at the time of manufacturing of a tire are remarkably improved.
2. Description of the Related Art
Due to the regulation regarding a spiked tire, development of a studless tire which satisfies the snow/ice grip performance of tires on snow or icy road surface has been strongly desired. Various studies have been done to the tread patterns of a tire, the rubber compound of a tread, or the like.
A so-called foamed tire (Japanese Patent Application Laid-Open No. Sho-62-283001), in which a rubber which mainly comprises a natural rubber/polybutadiene rubber and a carbon black is used for a foamed rubber layer provided at the tread, is well known as an example of improving the snow/ice grip performance by the tread rubber compound of a tire. This foamed tire has an excellent snow/ice grip performance, and when the tire is manufactured, the technology has solved a difficult technical drawback of controlling both vulcanization reaction and foaming reaction at the time of vulcanization.
In order to improve abrasion resistance of the foamed tire, it is preferable that the amount of a polybutadiene rubber used in the tread is increased. However, the polybutadiene rubber reduces the wet skid resistance of the tire. In order to solve the drawback, Japanese Patent Application Laid-Open No. Hei-7-258469 proposes a technology in which a combination of carbon black and silica is used as a filler.
However, silica particles tend to cohere together due to hydrogen bonding of silanol groups which are functional groups on the surfaces of the silica particles. For improving the dispersion of silica particles into rubber, the mixing time must be increased. When dispersion of silica particles into rubber is insufficient, a problem arises in that processability in processes such as extrusion and the like deteriorates due to the increase in Mooney viscosity.
Moreover, the surfaces of the silica particles are acidic. Therefore, there are problems in that basic substances used as vulcanization accelerators are absorbed such that vulcanization is not carried out sufficiently, and a sufficient modulus of elasticity is not obtained.
In order to solve these problems, various types of silane coupling agents have been developed. For example, an art in which a silane coupling agent serves as a reinforcing material is disclosed in Japanese Patent Application Publication No. Sho-50-29741. However, the use of a silane coupling agent as a reinforcing material is still insufficient for improving fracture properties, workability, and processability of a rubber composition to high standards. Rubber compositions in which a combination of silica and silane coupling agent is used as a reinforcing material are described in Japanese Patent Application Publication No. Sho-51-20208 and others. However, this method of using a combination of silica and silane coupling agent as a reinforcing material has a drawback in that flow of the uncured rubber composition is markedly inferior and workability and processability deteriorate, although reinforcement of the rubber composition can be remarkably improved and fracture properties are improved.
The drawbacks of the conventional technologies in which silane coupling agents are used arise due to the following mechanism. When the mixing temperature of rubber is low, the silanol group on the surface of the silica does not react sufficiently with the silane coupling agent, and as a result, a sufficient reinforcing effect is not obtained. Dispersion of the silica into the rubber is also inferior, and this causes deterioration of the low heat buildup property which is the strong point of a rubber composition containing silica. Moreover, some of the alcohol formed by the reaction of the silanol group on the surface of the silica and the silane coupling agent does not vaporize completely during mixing because of the low mixing temperature, and the residual alcohol in the rubber vaporizes during an extrusion process so as to form blisters.
On the other hand, when mixing is conducted at high temperatures of 150.degree. C. or more, the silanol group on the surface of the silica and the silane coupling agent sufficiently react with each other, and as a result, the reinforcing property is improved. Because dispersion of the silica into the rubber is also improved, a rubber composition having a good low heat buildup property is obtained, and the formation of blisters in an extrusion process is suppressed. However, in this temperature range, gelation of the polymer caused by the silane coupling agent takes place simultaneously, and the Mooney viscosity is markedly increased. Thus, processing in later stages becomes impossible in actuality.
Therefore, when a silane coupling agent is used in combination with silica, a multistep mixing must be conducted at a temperature lower than 150.degree. C., and marked decrease in productivity is inevitable. When the mixing at a low temperature is conducted, dispersion of silica and carbon black into the rubber is insufficient and abrasion resistance deteriorates.